The present invention relates to a liquid crystal optical device, and more particularly to a liquid crystal optical device having an optically active nematic liquid crystal composition capable of dielectric relaxation at low frequencies for high-speed responsiveness. The optical device is particularly well suited for use in a printing device including a liquid crystal light valve. It is known to use a liquid crystal device as a light modulation element in a printing device, a signal reading device, a signal converting device, a light signal switch, a device for adjusting a quantity of light, or simply as a light valve. (See, for example, Japanese Laid-Open Patent Publications Nos. 50-74340, 50-102343, 51-26053, 49-66149 and 51-80242.) The disclosed proposals have merely been indicative of ideas in principle, and have not been feasible as a practical matter because conventional liquid crystal materials and systems for driving the liquid crystals have only attained a speed of responsiveness (from several tens msec. to several hundreds msec.) and a frequency of repetition (from 10 to 1 Hz). Hence, they have not been completely satisfactory for the foregoing intended applications.
As semiconductor technology has advanced to render CPU's, memories and the like less costly, more and more people other than specialists handling EDP's are having a chance to use microcomputers and office computers and to deal with the printouts of their computers. Stated otherwise, there has been a great need for output processing at the same level as ordinary documents. This includes outputs expressed in Chinese characters and Kana (Japanese syllabary), word processors for the Japanese language being an example.
Although circuits and memories now do much to improve such a system in view of their ever reducing cost, printers as system output terminals require a resolving power as high as 32.times.32 dots suitable for printing Chinese characters which add to the cost of the systems. The printers should operate at speeds high enough to meet the high resolution requirement and compensate for low speed operation needed for high resolution printing. Devices available at the present time designed to meet the foregoing requirements are electro-photographic printers using lasers or OFT and multi-stylus electrostatic printers. Both of these are very expensive and the greatest item which increases, the cost of the systems. Thus, unavailability of desirable printers constitutes a bottleneck in popularizing the foregoing various systems in the market even though a need for such systems exists in the market. This situation basically holds true for high-speed facsimile, CRT hard-copiers and various other terminals.
With these points in view, the present invention is aimed at increasing high-speed responsiveness and provides a liquid crystal optical device which is quite effective for meeting all of the above-mentioned applications. The present invention also provides a printing device of the liquid crystal light valve type. Such devices have been considered to be difficult to produce as a practical matter due to a variety of technological problems.
High speed light valves for the foregoing and other applications are required to have the following characteristics: (1) They should be closed at high speeds; (2) They should be opened at high speeds; (3) They should be able to be opened and closed in short periods of time; (4) They should not allow much leakage of light when closed; and (5) They should have a high transmissivity of light when opened. With these characteristics in mind, conventional liquid crystal devices as light valves will be described to highlight their disadvantages. In order to achieve characteristics (4) and (5) above, twisted nematic liquid crystals are most effective for minimizing leakage of light. With the twisted nematic liquid crystals, leakage of light when the light valve is closed can be reduced to almost nothing by arranging polarizing planes of polarizers perpendicularly to each other. It is easy for some polarizers used to have 100/1 as a ratio of light transmission when the light valve is opened to light transmission when the light valve is closed. With this ratio, the light transmissivity while the light valve is opened is from 20 to 40%, which meets the condition of (5). Other liquid crystals than twisted nematic liquid crystals cannot satisfy the conditions (4) and (5).
For example, a dynamic scattering system, a guest-host system with a two-colored dye added to a nematic liquid crystal for a display, or a nematic-cholesteric phase-transition system allow much leakage of light, thus being unable to meet characteristic (4). Conditions (4) and (5) cannot be met by a system utilizing birefringence of liquid crystal molecules, since such a system only has an opening and closing function with respect to light of a particular wavelength, but not an opening and closing function with respect to all visible light.
Accordingly, it is desirable to provide a liquid crystal optical device which is capable of operating at high speed and will not allow much leakage of light when closed and has a high transmissivity of light when opened. Such a device will permit construction of an electro-photographic picture of high resolving power and high quality which will operate at high speed, is simple in construction, reliable in operation, of a small size and inexpensive to construct.